1. Field of the Invention
The present invention relates to an inkjet recording apparatus recording images on a recording medium by ejecting ink droplets, and in particular, to an inkjet recording apparatus in which ultrasonic waves are generated from an ultrasonic wave generation member, and the ultrasonic waves are focused to eject the ink droplets.
2. Description of the Related Art
An inkjet recording apparatus recording images by ejecting liquid ink as small droplets on a recording medium has a lot of advantages such that a direct recording on a plain paper and so on is possible, a low cost in an aspect of material consumption such as ink is realized, less noise, processes such as development, fixing, are not necessary. Accordingly, in recent days, utilization range is broaden out into industrial field such as a coating of a liquid electronic material, and a direct patterning, in addition to an application of printing images on a paper and so on.
Many methods are devised as the inkjet recording apparatus, but in particular, a method in which the droplets are ejected by using pressure of bubbles generated by heat of a heating element, a method in which droplets are ejected by a pressure pulse resulting from a displacement of a piezoelectric material, and so on are representatives.
However, there has been a problem that the ejecting of the ink droplets is disturbed because a concentration of ink is easy to occur caused by evaporation, volatile of the liquid ink solvent and a clogging occurs when a nozzle with a small diameter is used, in the inkjet recording apparatus in the above stated methods. Accordingly, it is necessary to provide an additional means such as a cleaning of the nozzle to prevent the clogging of the nozzle for a particularly high definition image recording, and the utilization range thereof has been limited such that a requirement to select and use an ink material with less clogging occurrences rises. Besides, a method in which fine ink drops are ejected without using a nozzle by vibrating a resilient member immersed in the ink by using the piezoelectric material so that the ink drops are ejected from a tip portion of the resilient member by the vibration, is known as the other method (for example, refer to JP-A 11-235820).
On the other hand, an ultrasonic type apparatus in which ultrasonic waves generated from an oscillator are focused and the droplets are ejected from a liquid surface by a sound pressure thereof is proposed (for example, refer to JP-A 2005-270929). Besides, an apparatus in which the ultrasonic method, focusing the ultrasonic waves generated from the oscillator and ejecting the droplets from the liquid surface by using the sound pressure, is used and constituted as a phased array head is also proposed (for example, refer to JP-A 8-99408). The above-stated ultrasonic method does not require the nozzle, being capable of ejecting the ink drops with very small diameter, and is suitable for high resolution recording. Besides, there is an advantage of less restriction for the usable ink and so on.
However, there are problems such that a large power is required to eject the ink drops because the ultrasonic waves are attenuated before the ultrasonic waves reach a focus point when ink with a large ultrasonic wave attenuation is used, or the ink drops cannot be ejected when the ink with the large attenuation is used, even though there is less restriction in the ink material and so on. Accordingly, an art is also known in which the ink is pressure-fed via a pore and so on, a thin layer of ink is formed on a surface of an ultrasonic wave propagation material to eject the ink from the thin film (for example, refer to JP-A 6-91890).
However, in the above-stated conventional art, the ink is supplied via the pore and soon, and therefore, the pressure is required to feed the ink, and there is a problem that it is difficult to supply the ink evenly to a whole head. Besides, there also is a problem that it is difficult to realize a stable ejecting caused by a liquid surface fluctuation of ink resulting from the ejecting of ink or the evaporation of ink.